This disclosure relates generally to coating systems for protecting metals. More specifically, it is directed to slurry coating compositions for providing aluminum enrichment to the surface region of a metal substrate.
In the case of turbine engines, the substrate is often formed from a superalloy comprising cobalt or nickel. The term “superalloy” refers to complex alloys comprising cobalt or nickel that include one or more other elements such as aluminum, tungsten, molybdenum, titanium, and iron. The aluminum component imparts environmental resistance to the alloys, and can also improve their precipitation-strengthening properties.
Superalloy substrates are often coated with protective metallic coatings. One example of a protective metallic coating is an aluminide material, such as nickel-aluminide or platinum-nickel-aluminide.
If such superalloy substrates are exposed to an oxidizing atmosphere for an extended period of time, they can become depleted of aluminum. One method for increasing the aluminum content of the superalloy substrate is sometimes referred to as “aluminiding” or “aluminizing.” The aluminum can be introduced into the substrate by a variety of techniques. In the “pack aluminiding” process, the substrate is immersed within a mixture (or pack) containing the coating element source, filler material, and a halide-activating agent. At high temperatures, for example at temperatures about 700° to about 750° C., reactions within the mixture yield an aluminum-rich vapor, which condenses onto the substrate surface. During a subsequent heat treatment, the condensed aluminum-based material diffuses into the substrate. In another method, the aluminum coating is applied by means of a high temperature chemical vapor deposition or any other gas phase means. These methods use high temperatures to vaporize the aluminum. In addition, aluminum is deposited on all exposed surfaces. The portions of the article not desired to be coated are protected with a high temperature resistant masking material. The masking process is time-consuming as a result of which vapor-phase methods are costly and time-consuming.
Internal passages are generally present in gas turbine components to allow for the passage of cooling air. As gas turbine temperatures have increased, the geometries of these cooling passages have become progressively more circuitous and complex. However, parts requiring internal aluminizing are treated with a vapor phase aluminizing process, which causes the parts to become expensive.
It would therefore be desirable to use a coating that can be easily and economically prepared and can further be applied to selected surfaces of an article. It would further be desirable to have a composition and method to facilitate aluminizing of internal cooling passages without requiring vapor phase aluminizing processes.